Catalytic reforming of straight-run naphtha in the presence of a platinum-alumina catalyst has been adopted widely as a process for preparing high-octane gasoline on a commercial basis. As naphtha to be fed to this catalytic reforming, a fraction boiling in the range from 70.degree. to 180.degree. C. is used for the preparation of automotive gasoline while a fraction boiling in the range from 60.degree. to 150.degree. C. is used for the preparation of BTX.
With this type of catalytic reforming, the conversion of feed naphtha to aromatic hydrocarbons decreases markedly as the number of carbon atoms in the naphtha decreases and this makes it difficult to prepare high-octane gasoline blending stock from light hydrocarbons mainly comprising of paraffins and/or olefins having 2 to 7 carbon atoms. For this reason, light hydrocarbons find limited uses as raw materials for the production of petrochemicals and city gas under the present conditions.
In consequence, technologies for preparing high-octane gasoline blending stock from light hydrocarbons are drawing attention in recent years as they enhance the value added of light hydrocarbons and accomodate increasing gasoline consumption.
The technologies proposed thus far for the preparation of high-octane gasoline blending stock from light hydrocarbons utilize a variety of catalysts: for example, hydrogen type ZSM-5, Ga-impregnated and/or Ga-exchanged hydrogen type aluminosilicates of MFI structure, hydrogen type gallosilicates of MFI structure, steam-modified crystalline galloaluminosilicates obtained by treating hydrogen or ammonium type galloaluminosilicates of MFI structure with steam [Kohyo Tokkyo Koho 60-501, 357 (1985)], and hydrogen type aluminogallosilicates of MFI structure described in an invention of the present inventors, Japan Kokai Tokkyo Koho No. Sho 62-254, 847 (1987).
However, hydrogen type ZSM-5, Ga-containing aluminosilicates, and crystalline gallosilicates are inferior to hydrogen type crystalline aluminogallosilicates as catalyst for the production of aromatic hydrocarbons. On the other hand, the aforesaid steam-modified crystalline galloaluminosilicates have structural defects as steam eliminates aluminum together with gallium from the skeletal structure during the modification with steam; they are likely to suffer permanent degradation of their catalytic activity in the course of their prolonged use and they are not yet satisfactory for commercial use. The aluminogallosilicates described in Japan Kokai Tokkyo Koho No. Sho 62-254, 847 (1987) do not show noticeable detachment of gallium from the crystal skeletal structure even in an atmosphere of hydrogen; however, they yield gradually less and less of aromatic hydrocarbons over a prolonged period of time with the selectivity to aromatic hydrocarbons being not sufficiently high and they still need to be improved in order to be commercially acceptable.
In the reactions directed to the preparation of high-octane gasoline blending stock from light hydrocarbons, it is important to maintain the yield of aromatic hydrocarbons at a high level stably over a prolonged period of time. Those catalysts which show a high activity in the initial phase but degrade easily, for example, suffer a severe loss of the activity by deposition of coke, are not suitable for commercial applications. Ideal catalysts are those which show a high activity initially and maintain that activity for a long period or to have a long life.
The reactions for the preparation of high-octane gasoline blending stock from light hydrocarbons in the presence of zeolite catalysts generally detest the presence of moisture.
The reaction of this kind is usually conducted at high temperature in the range from 300.degree. to 700.degree. C. If any moisture into contact with the catalyst at such a high temperature, it extracts the aluminum atoms in the catalyst or it causes dealumination and the resulting structural defects in the catalyst at times make it impossible to maintain the catalyst activity at a specified level. For this reason, a key point in the operation of a process involving the reaction of this kind is how to maintain the system dry, which is essential for the maximization of catalyst life. In consequence, a variety of devices have been installed for the removal of moisture; for example, one process adopts a dryer and another a device for the separation of moisture by adsorption.
In contrast to this, what Japan Tokkyo Koho No. Hei 1-47, 224 (1989) proposes for the enhancement of the activity of acid type zeolite catalysts is to contact zeolites with water under specified conditions or to effect such contact with water in the presence of ammonia. According to this process, acid type zeolite catalysts having a silica to alumina mol ratio of at lease 12 and a control index of 1 to 12 are contacted with water under the conditions of time, temperature, and partial pressure of water satisfying a specific relationship. In particular, this process is understood to be designed for the improvement of acid activity/decomposition activity attributable to acid sites of zeolites. In the aromatization reaction of light hydrocarbons such as relating to this invention, however, it is necessary to suppress the decomposition reaction accompanied by the formation of light gases such as methane and ethane and let the activity of dehydrocyclization take precedence of that for the decomposition reaction. Therefore, it is not possible to let the zeolites of this process as they are serve as crystalline aluminogallosilicates suitable for the preparation of high-octane gasoline blending stock from light hydrocarbons. Moreover, even if the conditions of time, temperature, and partial pressure of water are established on the basis of the aforesaid specific relationship, they are difficult to realize in practice. In addition, the realization will need special equipment, which is a disadvantage from the economical viewpoint. Furthermore, the treatment with steam surely improves the initial activity of the catalyst, but it causes dealumination in a portion of the catalyst structure and adversely affects the catalyst life.
The present inventors have conducted extensive studies to overcome the aforesaid problems and found that, in the reaction for the preparation of high-octane gasoline blending stock from light hydrocarbons in the presence of a catalyst composition containing crystalline aluminogallosilicates as catalyst component, the use of ammonia-modified crystalline aluminogallosilicates obtained by treatment with ammonia under specified conditions as such catalyst component markedly improves not only the initial catalyst activity but also the catalyst life and advantageously produces high-octane gasoline blending stock. That is, said crystalline aluminogallosilicate catalysts with a dual function of decomposition and dehydrocyclization can be improved markedly in their dehydrocyclization activity and life by the modification with ammonia. This invention has been completed on the basis of this finding.
Accordingly, it is an object of this invention to provide a process for preparing high-octane gasoline blending stock which comprises modifying crystalline aluminogallosilicates for use as catalyst component, preparing therefrom catalysts of high initial activity and long life, and preparing high-octane gasoline blending stock advantageously from light hydrocarbons with the use of said catalysts.
Another object of this invention is to provide a process for preparing high-octane gasoline blending stock which comprises modifying crystalline aluminogallosilicates in such a manner as to make them serve as a component of the catalyst for the preparation of high-octane gasoline blending stock from light hydrocarbons while enabling said catalyst to achieve high initial activity and long life and produce advantageously high-octane gasoline blending stock from light hydrocarbons.
A further object of this invention is to provide a process for preparing high-octane gasoline blending stock advantageously from light hydrocarbons, particularly those mainly comprising of paraffins and/or olefins having 2 to 4 carbon atoms.